[DOC] Fix outdated results in examples

Some example results in the docs no longer match what Ruby prints:

* Array#flatten and Array#flatten!: Set#inspect now shows Set[6, 7]
* Module: Mod.constants also includes Math::DomainError
* Module#const_get and Module#const_set: Floats are shown with full
  precision
* Complex#imag: sin(PI/4) is 0.7071067811865475
* String#<<: missing # in #<Encoding:UTF-8>
* Enumerable#inject: typo #=>>

enum.c

@@ -1021,7 +1021,7 @@ ary_inject_op(VALUE ary, VALUE init, VALUE op)
  *      end
  *
  *      position = "nnneesw".chars.reduce(Turtle.new, :move)
- *      position  #=>> #<Turtle:0x00000001052f4698 @y=2, @x=1>
+ *      position  #=> #<Turtle:0x00000001052f4698 @y=2, @x=1>
  *
  *  <b>Third Shortcut: Reducer With no Initial Value</b>
  *